Supposedly, according to sexTV, this is becoming more and more popular among couples etc. I can understand it as the anus has a tremendous amount of nerve endings.
IMHO tho this is not something I would personally enjoy. After about 3 colonoscopies, and other exams, this part of my body is "exit only".
Now, watching or being a part of, a woman doing another woman, that is a whole new ballgame!!! YEEEHAWWW
As for pointers I would assume it would be the same as a man doing a woman anally:
1) lube, LOTS of lube, you can't have enough LUBE (did I mention lube?) The anus doesn't produce ANY natural lubricant so without additional support........
2) patience, you can't just push your way in like getting on a crowded subway
3) Don't pull out all the way, the worst part is the re-entry
4) did I mention lots of lube?
5) if he' or she's a newbie, don't buy that 14" 3" diameter strap on. WHile a woman's vagina is meant to expand to allow the passing of a babie's head, the anus ISN"T (unless one eats a LOT of mexican food first!! LOL)